Ceramics based on beryllium oxide (BeO) are well known and are valued for their high thermal conductivity combined with high dielectric strength, which, in combination with their mechanical properties, makes them useful as substrates for electronic circuits though this is only one of many commercial applications for the product.
Conventional beryllium oxide ceramics for such end uses typically contain 99.5% by weight BeO and have added 0.3% by weight magnesium oxide (MgO) and 0.2% silica (SiO.sub.2). This forms a phase which aids densification of the green body and bonds together the beryllium oxide (BeO) grains. The density of ceramics formed on sintering this composition is usually in the range 2.85-2.91 grms cc resulting in a remnant closed porosity of 3.3-5.3%. The grain size of the ceramic is typically 20-30 microns with the remnant porosity present both within the grains and also at the grain boundaries. This means that the smoothness of the surface which can be obtained by polishing is limited and on the microscopic scale now involved in electronic circuits the surface is liable to present fissures with the attendant risk of gaps in thin metallic film circuits which pass over such fissures.
Previously SiO.sub.2 has been added with MgO to reduce the sintering temperature of BeO from approximately 1800.degree. C. to more manageable and economic levels namely around 1500.degree. C. and also to act as a grain controller i.e. helping maintain an even grain size.
Beryllium oxide ceramics are often used in electronic circuits as wafers e.g. 0.5 mm thick and 4 1/2.times.4 1/2 inch (11.4 cms.times.11.4 cms) square. High strength is called for, and a recent change in circuit fabrication has led to the desire for higher strength still. Thus before this change the method for attaching the electrical device to the beryllia substrate was by forming an intermediate nickel layer on the ceramic wafer. This arrangement gave a thermal conductivity reduced by about 10%. The technique now preferred is to clamp the BeO wafer to the metallic device giving a direct pressure contact.
It is desired to have as high a density as possible. The theoretical density for BeO is 3.01. The commercial grade of BeO ceramic mentioned above has a density of 2.85-2.91, this varying with a Particular batch of oxide received from the suppliers. For a typical conventional BeO ceramic containing 0.3% MgO, 0.2% SiO.sub.2, 99.5 % BeO plus trace elements having a theoretical density of 3.01, a density in the range 2.85-2.91 is 94.7% to 96.6% of the theoretical density. The remnant porosity which can be up to 5.3% by volume, is particularly important since it is mainly responsible for the limitation of properties of importance in design and fabrication of devices using beryllia ceramic.
Thus an increase in density may also be expected to result in a superior material, the removal of flaws associated with the porosity giving rise to a ceramic having a higher fracture stress, with less tendency to chipping and grain displacement on machining.